1. Field of the Invention
This invention relates to a thermochemically treated silicon carbide fiber derived from organosilicon oligomeric and/or polymeric precursors including doped oligomers and polymers. The invention also relates to the process of thermochemically treating the fibers by annealing the oligomeric and/or polymeric derived silicon carbide fibers in an excess of carbon particles under an interactive gas atmosphere such as nitrogen gas.
2. Description of Related Art and Problems in the Art
Composites are structural materials formed by the combination of several materials. Composites are used where there is a need for a lighter weight structure which must be stronger, tougher, and able to survive harsher environmental and temperature cycling conditions which cannot be met by traditional unreinforced single materials or alloys. Composites provide the opportunity to design and fabricate structural material having specific chemical, thermal, mechanical, and electromagnetic properties. Composites can be fabricated from plastic, ceramic and/or metal materials.
Flaws of any size are likely to cause failure of loaded brittle ceramic or metal materials. This flaw intolerance tends to limit the use of brittle materials, particularly ceramics, in situations which require structural reliability.
Ceramic materials can be strengthened by the addition of fibers. The fibers may be composed of many materials depending on the particular composite and fabrication and operating conditions for that composite. Ceramic and metal composites can be strengthened by the addition of ceramic fibers formed by any of the very well known means or purchased from commercial sources.
Theoretically, the appropriate inclusion of fibers in a ceramic matrix, particularly high strength ceramic fibers, should increase the fracture toughness and strength of the ceramic product. In fact, it has been noted that the performance of ceramic fiber reinforced materials has fallen short of expectations.
Ceramic and metal composites are formed by hot pressing, rolling, molding, sintering, and other techniques in which the composites materials are subjected to high temperatures and pressures. These conditions tend to degrade the strength of known ceramic fiber materials, particularly those derived from organosilicon oligomeric and/or polymeric precursors. Both chemical and mechanical reasons have been identified for the poor results of oligomeric and/or polymeric derived reinforcing fibers. In some cases, chemical incompatibility between the fiber and the matrix results in such a poor bond between the matrix and the fiber that no load can be transferred to the fiber. In other cases the fiber and matrix react together causing too strong of a bond to form at the fiber/matrix interface. The fiber may also lose strength during the various densification processes used to form the composite. These changes in the fiber and the matrix ultimately lead to a weakening of the composite material as a whole.
The thermochemical instability of fibers derived from organosilicon oligomer and polymer precursors such as silicon carbide are a major limitation in the choice of processing conditions and matrix materials for the end product composite. The exact reasons for the degradation of, for example, oligomer and polymer-derived silicon carbide fibers is not completely understood. Degradation products such as silicon dioxide and carbon dioxide have been identified being exuded by the fibers. Some authors, such as Anderson and Warren, in "Silicon Carbide Fibers and Their Potential Use in Composite Materials, Part I Composites," Vol. 15 (No. 1) p. 16-24 (1984), propose crystallite growth, micro-porosity formation and (flaw) growth as possible reasons for fiber degradation.
Fiber surface coatings and fiber surface treatments have been proposed as means to prevent fiber degradation and to make the fiber more chemically compatible with the matrix. In U.S. Pat. No. 4,340,636 Debolt et al. suggest vapor phase treating a stoichiometric CVD silicon carbide filament to create a carbon rich silicon carbide surface which is more chemically compatible with organic polymers and alumina materials when forming composites with those materials.
Bender et al. have shown that a coating of boron nitride (BN) on the surface of a polymer-derived silicon carbide fiber strengthened the fiber matrix composite and they showed that the coating acted as a diffusion barrier to protect the fiber from oxidation or from volatilizaton from reactions with the matrix, Bender et al. "Effect of Fiber Coatings and Composite Processing on Property of Zirconia-Based Matrix Silicon Carbide Composites," American Ceramic Society Bulletin, Vol. 65, No. 2, Feb. 1986, p. 363-369.
Others have also suggested the use of boron nitride as a coating to strengthen ceramic fiber composites. Singh and Bruhn "Effect of Boron Nitride Coating on Fiber-Matrix Interactions" Ceram. Eng. Sci. Proc., 8 (7-8) pp. 636-643 (1987); Rice et al., "The Effect of Ceramic Fiber Coating on the Room Temperature Mechanical Behavior of Ceramic-Fiber Composites", Ceram. Eng. & Sci. Proc., 8th Annual Conference, July-August 1984, published by the American Ceramic Society, Columbus, Ohio, 1984; Lewis and Rice, "Further Assessment of Ceramic Coating Effects on Ceramic Fiber Composite", NASA Conf. Publ. 2406, Proceedings of a Joint NASA/DOD Conference, Jan. 23-25 1985.